Re: [APML] Exposure time with filter

["Jim Ives" <flyboy9990@earthlink.net>]

Robert do you have any idea off hand what the % of transmission is through a
Wratten #92 at the Ha line?
Jim Ives
----- Original Message -----
From: "Robert Reeves" <reeves10@swbell.net>
To: <astro-photo@seds.org>
Sent: Wednesday, November 27, 2002 6:58 PM
Subject: Re: [APML] Exposure time with filter


>
>
> > Dear All,
> >
> > I send this question again as you probably didn't receive my first one.
> > I wanted to buy a dark red filter in order to prevent light pollution
> > but filters like wratten #92 has a factor 8 as absorbtion. Is this true
> > that I need to expose for a time 8 times longer than before? So an 10
> > minutes exposure becomes an 80 minutes exposure? I need to understand
> > this because if this is true I cannot do that way as I don't have
> > autoguide....:-/
> >
> > thanks
> > Nicola
>
> Nicola,
>
> I assume you are getting the #92 filter for H-Alpha work with
red-sensitive
> B&W film?  If so, then consider the filter nearly transparent to the
H-Alpha
> wavelength you want to record.  Here is a simplified explanation that
ignors
> some other important emission lines from emission nebulae.  The #92 filter
> appears dark to the eye and needs a filter exposure correction in daylight
> for terrestrial photography because it blocks all wavelengths except red.
> With most of the visible spectrum removed, the remaining red wavelengths
> will need longer exposure to form an image on film.  But in
> astrophotography, it will pass nearly all the red H-Alpha wavelength you
do
> want while blocking the light pollution you don't want.  Since the H-Alpha
> is the primary wavelength building up the image of an emission nebula,
that
> is the very wavelength you wanted in the first place.  The bluer
wavelengths
> with the light pollution were just fogging your film.  With the filter
they
> will be blocked, while the desired red wavelength passes.  Because the
light
> pollution is not fogging the film anymore, the filter allows longer
> exposures which record deeper H-Alpha detail.  You can expose up to ten
> times longer than without the filter.  But this is not to compensate for
the
> "filter exposure factor", it is because light pollution is radically
reduced
> and not rapidly fogging the film.
>
> Remember that when using the red filter, the lens infinity focus will
shift
> slightly.  The lens will have to be focused with the filter in place.
>
> Robert Reeves                      reeves10@swbell.net
> 520 Rittiman Rd.                   www.robertreeves.com
> San Antonio, Texas 78209    210-828-9036
> USA                                     29.484  98.440  200 meters
>
>
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